1. Field of the Invention
The invention relates in general to a technique for observing a semiconductor device microscopically, and more particularly to a technique that uses reagent to aid in the microscopic observation of a semiconductor device.
2. Description of the Related Art
In a semiconductor device, a tungsten plug is usually used to connect two conducting layers to form a complete circuit. Therefore, the quality of the tungsten plug affects the quality of the semiconductor device. To control the reliability of the semiconductor device manufacturing process, the fine structure of a sample semiconductor device should be observed on a microscopic level. For example, the scale of the tungsten plug and the thickness of a glue layer can be learned by observing the device with a microscope.
A conventional semiconductor device may have a contact window of a tungsten plug that is larger than 0.5 .mu.m. The tungsten plug can be cut at the center. By etching with a glass reactive solution or a polysilicon reactive solution, for example, a solution of hydrofluoric acid (HF), a solution of hydrofluoric acid and acetic acid (CH.sub.3 COOH), or a solution of hydrofluoric acid and aqua fortis, the orientation of the grain of the tungsten plug and the structure and thickness of the glue layer 12 can be observed with a microscope.
Because of the rising level of integration, the width of the contact window of the tungsten plug may be less than 0.5 .mu.m. Therefore, the tungsten plug 20 cannot be cut at the center to show its grain orientation clearly.
To reduce the problem stated above, the center of the tungsten plug 30 is polished after cutting through the tungsten plug. Next, an etching process is performed with an etchant such as those noted above. Finally, the semiconductor device is observed using a scanning electron microscope (SEM). However, the surface of the tungsten is difficult to observe with respect to the grain structure of the tungsten plug and the thickness of the glue layer because the polishing planarizes the surface of the tungsten.